Neuroblastoma, one of the most common forms of childhood cancer has been treated clinically with various antimetabolites used singly, sequentially, or in combination. This tumor has been cloned from the mouse and adapted to tissue culture. It is the objective of this research proposal to explore the effectiveness of a series of new folic acid analogues by studying the molecular mechanisms of resistance in this experimental system. We will compare the original drug sensitive C-1300 neuroblastoma cell line with a number of resistant cells lines, derived by exposing the parent line to one of a series of antifolates and other antimetabolites involving metabolic pathways requiring reduced folates. By correlating the degree of resistant to growth inhibition in these lines with changes in their ability to transport or concentrate these drugs or with marked changes in the in vitro activities of enzymes affected by these drugs, emphasizing (thymidylate synthetase) and (dihydrofolate reductase), the mechanisms of resistance of each line may be ascertained. These studies are intended to provide a better understanding of the patterns of drug resistance, allowing more effective chemotherapy regimens to be formulated.